WASHINGTON — On the eve of a showdown vote, House Democrats and the White House floated possible compromises today on President Reagan's plan to send $100 million in military aid to rebels fighting Nicaragua's leftist government.

The behind-the-scenes maneuvering came as Reagan appeared still short of a majority in the Democratic-controlled House and as leading Democrats grudgingly conceded that the President will ultimately win some aid for the contra rebels.

Trying to gain additional support, the White House sent to Capitol Hill a draft executive order that committed Reagan to pursuing a peaceful settlement of the Nicaraguan conflict but giving the rebels the $100 million in aid only if talks fail in 90 days.

Rep. Michael D. Barnes (D-Md.) said the proposed compromise "doesn't really change anything. . . . It's an obvious indication that the Administration doesn't have the votes on the merits and they're grasping for ways to get some votes."

'Tonkin Gulf Vote'

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) said the House vote, scheduled Thursday, was a "Tonkin Gulf vote," a reference to the 1964 resolution that enabled then-President Johnson to introduce U.S. troops in Vietnam.

As the House renewed its acrimonious debate on Nicaragua, O'Neill said, "I see this leading to war. . . . I see a quagmire down there."

O'Neill said the Democrats maintained a 10-to-15-vote lead and had seen "no erosion" over the past days despite an intensive personal lobbying campaign by Reagan.

O'Neill also announced that if the President's plan is defeated, he will schedule votes on various compromises in mid-April.

O'Neill said he opposes all aid to the contras, but acknowledged that Rep. Dave McCurdy (D-Okla.), a moderate, is preparing a compromise plan and wants a chance for a vote on it after action on Reagan's plan.

'None Agreed To'

At the White House, spokesman Larry Speakes ruled out compromise on the "fundamentals" of the $100-million package that includes both military and logistical aid, but added that changes were possible on the timing of aid deliveries to allow more time for diplomatic efforts.

"There has been a suggestion of an executive order," Speakes said. "There have been suggestions of letters. But none has been agreed to."

Steve Patterson, an aide to McCurdy, said McCurdy and Rep. Rod Chandler (R-Wash.) were shown a draft executive order containing Reagan's possible compromise. Other sources said the President is willing to withhold much of the military aid for 90 days while trying to reach a negotiated settlement.

However, Patterson noted that an executive order "expresses the intent of the President on the day that it is signed, but the President can rescind the order at any time."

Military Aid in 30 Days

Other congressional sources said McCurdy's compromise bill would permit part of the money to go immediately for logistical aid with military aid released in 90 days unless Congress votes separately to withhold it.

Under special rules governing Reagan's $100-million plan, no floor amendments are possible, meaning that compromises can only come in the form of promises by Reagan on how the money would be handled or by later legislation.

In the House debate, Rep. Bob Livingston (R-La.) urged passage of the aid as necessary to stop the spread of communism in Central America. "Those Sandinistas, those communists (in Nicaragua), are closer to me in Louisiana than I am to New York City," he said.

Rep. David R. Obey (D-Wis.) said, "The United States of America, with our great tradition, should not be in the business of funding a grinding, low level dirty little war under conditions which at best we can play to a tie."